Arrangement relating to telephone, telegraph, and signal systems



y 8-- s. D. VIGREN El AL 2,118,279

ARRANGEMENT RELATING TO TELEPHONE, TELEGRAPH, AND SIGNAL SYSTEMS Filed July 16, 1935 5 Sheets-Sheet 1 S-fen Dan/ e/ l l g dfl Hc/ge 5 05f INVENTORS BY WAN c.

751w ATTORNEY- y 1938. s. D. VIGREN ETIAL 2,118,279

ARRANGEMENT RELATING TO TELEPHONE, TELEGRAPH, AND SIGNAL SYSTEMS Filed July 16, 1935 3 Sheets-Sheet 2 5/2 Uam c/ 1 49/12 Hrs/ye F057 INVENTOR5 tzulv ATTORNEY.

May 24, 1938. s. D. VIGREN ET AL ARRANGEMENT RELATING TO TELEPHONE, TELEGRAPH, AND SIGNAL SYSTEMS Filed July 16, 1935 3 Sheets-Sheet 3 Helge. Rt s+ ha ATTORNEY.

Patented May 24, 1938 ARRANGEMENT RELATING TO TELEPHONE,

TELEGRAPH, AND SIGNAL SYSTEMS Sten Daniel Vigren, Stockholm, Sweden, and Helge Root, Mexico, D. F., Mexico 1935, Serial No. 31,582 July 24, 1934 Application lficlaims.

July 16,

Sweden The present invention relates to new and useful improvements in telephone, telegraph and signaling systems and particularly to systems of the type using trunk lines for two-way traffic between exchanges or stations.

In two-way trunk trafllc over the same line between, for instance, two automatic telephone exchanges or between two telegraph stations difficulties have always been encountered heretofore in obtaining distinct and reliable transmission of signals between said exchanges, stations, etc.

These difficulties have especially been troublesome in view of the following circumstances:

(a) Different ground potentials at the intercommunicating exchanges or stations, which circumstance causes a current to flow in a grounded line from one exchange or station to the other. (b) An induced current may arise flowing from the ground of one exchange or station to that of the other, said current passing through the line. The voltage of the grounded battery of one exchange or station may be different from that of the battery of the other exchange or station. In such a case, a current will constantly flow through the connecting line if both batteries are grounded.

In order to avoid these difliculties, especially under idle (non-operating) circuit conditions, in exchanges and stations using grounded batteries, which are connected to both ends of a trunk line, several remedies have been proposed, but the devices used were costly, cumbersome and not entirely satisfactory;

Formerly, when signals were sent out over such a trunk line in one direction only, false operation ofthe relays in the line was prevented by marginal adjustment of the relays. However, since the introduction of loop-signaling and two-way traflic over the same line, the magnitude of thecurrents employed rendered such arrangements impracticable.

When a. single line with ground return is used, disturbing ground currents may often interfere with the receiving devices to such an extent that the transmission of impulses becomes impossible. In the northern hemisphere disturbing ground currents are frequently caused by so-called magnetic storms paralyzing telegraph trafllc for several days.

According to the present invention these difllculties are eliminated and signal systems may be used for two-way trafllc without interference of disturbing ground or other parasitic currents, or differences of potentials between two points of a signaling system, by connecting the calling or receiving devices, or both, in series with rectifiers,

for instance, copper-oxide rectiflers, in such a manner that disturbing ground currents are suppressed or are permitted to circulate only in a direction that will not materially affect normal v signals.

Sometimes these rectiflers must not remain connected with the line after a call or signal has been transmitted. According to this invention the rectifiers are disconnected when the call or the series of impulses have been transmitted and reconnected to the trunk line when the latter is restored to normal, i. e., the condition prevailing before the initiation of the call.

Calls over two-way telephone trunks are, according to this invention, established over single line and ground return or over a metallic loop circuit, whereby the danger of outside interference is further reduced due to the interconnection of rectiiiers as aforementioned.

The preferred form of the present invention provides connection of rectiflers in series with a receiver and/or a current source in a line signal system. The line comprises a grounded current source or a grounded receiver or both at one or both ends of the line. The line serves traillc in both directions. The rectiflers are used in order to prevent under idle- (non-operating) circuit conditions disturbing currents from entering the system, through the ground connections thereof.

The invention also provides for disconnecting in known ways the rectifier associated with the receiver at a station after the receipt of a call at said station, and the reconnection of said rectifler after the line is restored to normal.

Another form of the invention consists in the arrangement that after the receipt of a call at a station, the rectifier associated with the receiver at that station stays in the circuit during the transmission of impulses in order to bar undesirable disturbing currents during the idle moments 40 of intervals between each signal received or transmitted. Thereafter the rectiflers are disconnected and they are again re-connected after the line is restored to normal.

The invention also provides means for disconnecting a rectifier at a station from the line during the transmission of a call and during each transmitted signal impulse and reconnecting the same during idle line conditions.

The invention and its application may, of course, be modified without departing from its spirit. It can be used in automatic telephone systems between the different selecting stages when trunk lines interconnect telephone exchanges haidng diierent battery voltages, for fillstance, 24, 36, 48 volts, etc.

According to the present invention rectifiers are used connection with two-way one-wire trunks for duplex and multiplex signal channels. l'he rectifiers are connected in series with the batteries or generators in such a manner that the rectifiers follow the batteries when the polarity of the current sources change poles with respect to line and ground. The balancing networks or resistances at a station are also generally connected in series with rectifiers to ground, a polechanger being provided to change the direction of the current through the balancing resistances when the polarity of the current sources changes with respect to the line.

Instead of introducing a double pole changer, the same result may be obtained by installing a single pole-changer. In this case a duplicate balancing resistance is provided in series with another rectifier and one or the other of the balancing resistances with its rectifier is connected to the line in such a manner that disturbing currents will either be barred or given such a direction that the fidelity of the signals transmitted will not be impaired. This invention is applicable also to one-wire simplex telegraph and signaling systems, e. g., open or closed Morse telegraph circuits.

According to the invention, in closed circuit systems, where current passes through the line when no signals are transmitted, and the circuit is interrupted when signals are transmitted, rectillers are connected in series with the ground connection at one or both offices in such manner that the operating current is allowed to pass and disturbing ground currents in the opposite direction are barred.

Should the disturbing currents flow in the same direction as the operating current, an increase of the operating current usually will not disturb operations the same way as current in the opposite direction. It is always easier to adjust an electromagnet for strong than for weak impulses.

In order entirely to avoid disturbances by stray currents, e. g., caused by magnetic storms, according to the present invention the polarity of the operating current sources connected to the line and rectifiers are so chosen that the operating currents oppose the disturbing currents, whereby the rectifier will bar out the disturbing currents entering in the system.

In open circuit systems the rectifiers may be connected in series with receivers at both offices, whereby each rectifier allows the received operating current to pass, but bars foreign currents in the opposite direction.

In order to avoid interference from stray currents, the polarity of the operating current sources connected to the line may be so chosen that no stray current can enter the system.

In the following, a few embodiments of the invention will be described with reference to the drawings in which Figs. 1 and 2 illustrate two automatic telephone exchanges, respectively; Fig. 3 shows an open circuit Morse telegraph system; Fig. 4 shows a closed circuit Morse telegraph system; Fig. 5 illustrates a duplex telegraph system. Each half of a figure represents a different mice (telephone or telegraph), the two ofiices being interconnected by a one or two-wire two-way trunk.

Fig. 1 shows two automatic telephone exchanges S1 and S2 which are connected with each other over the two-way trunk line L.

re ale For instance a call from a subscriber in exchange S1 reaches the line Lin the following way: When the subscriber lifts his receiver from the hook, the subscribers line circuit is closed and operates a line relay (not shown) in known way. The subscriber dials in known manner a number corresponding to exchange S2. By means of a group selector (GV) multiple bank the sul scriber is thereupon directly or indirectly connected torelay F1, which receives the same number of impulses dialled by the subscriber.

Relay F1 is thus closed over the following circuit: Grounded pole of battery B1, upper winding of relay F1, line a, over GV-multiple bani: to subscribers instrument and back (not shown on the drawings), over line b, lower winding of relay F1 negative pole or" battery E1. Armature of relay F1 is operated and contacts 2, 3 and l, El are closed. Through the closing of said contacts 2 and 3, relay F3 is operated in the following way: Ground 'pole of battery B1, contacts 2 and 3 of relay F1,

windng of relay F3, negative pole of battery B1. Upon operation of the relay F3, its armature is operated and the following contacts are closed: 2, 3; 1,5: and6,l.

When contacts 6 and I of relay F3 are closed, relay F4 is also operated by the following circuit: Ground pole of battery B1, contacts I and 6 of relay 1 2, winding of relay F4, negative pole of battery B1; whereby relay F4 will be energized and its armature operates, closing the following contacts: 2, El; 5, 6; and I, 8. Simultaneously, contacts i, 2 and 3, 5 are opened, disconnecting group-selector GV1 of exchange S1 from line L for incoming traflic from exchange S2.

As soon as contacts 2, 3 of relay F4 is closed, the following circuit is established: Negative pole of battery B1, contacts I, 2 of relay F5, contacts 2, 3 of relay F 1, contacts I, 2 of relay F14 upper winding of relay G11, rectifier A2, to ground of battery B2 and then back to ground of battery B1.

Relay G11 is energized closing its contacts I, 2. Closing of said contacts closes the operating circuit of relay G12 as follows: Grounded pole of battery B2, contacts l, 2 of relay G11, winding of relay G12, negative pole of battery B2. Relay G12 operates its armature and closes its contacts I, 2.

The following circuit is then closed: Negative pole of battery B2, contacts 2, I of G12, lower winding of relay G11, contacts 4, 5 of relay F14, contacts 5, 6 of relay F4, contacts 5, 6 of relay F5, winding of F5, contacts I, 8 of relay F4, grounded pole of battery B1 and back to grounded pole of battery B2.

Relay F5 is thereby operated, and opens its contacts I, 2, thereby disconnecting negative pole of battery B1 from the line L; at the same time con-. tacts 6, 1 of F5 enclosed and relay F5 receives holding current as follows: Negative pole of bat tery B1, contacts 1, 6 of relay F5, winding F5, contacts 'I, 8 of relay F4, ground pole of battery B1.

The call to exchange S2 is completed over the following now established loop-circuit:

Ground of battery B2, rectifier A2, upper winding of relay G11, contacts I, 2 of relay F14, upper Wire of line L, contacts 2, 3 of relay F4, contacts 2, 3 of relay F3, upper winding of relay F1 (no armature shown on this relay) contacts 3, 4 of relay F5, contacts 5, 4 of relay F1, contacts 6, 5 of relay F4, the lower wire of line L, contacts 5, 4 of relay F14, the lower winding of relay G11, contacts I, 2 of relay G12, minus pole of battery B2.

The transmission of impulses from subscriber of exchange S1 to exchange S2 can now proceed, said impulses being primarily received by relay F1 in exchange "81, which relay F1 opens and closes are received in exchange S: by relay G11 of groupselector GVa, which selector completes the call in known ways. 7

For each impulse, and when the relay F1 is deenergized, its contacts I, 2 are closed. This closing of contacts I, 2 of relay F1 closes a circuit,

I, which energizes relay Fe according to the following circuit: Ground pole of battery B1, contacts I, 2'01 relay F1, winding of slowacting relay F's,

contacts 5, 4 of slow acting relay F3, negative pole ,of battery B1. Upon energization of relay Fa, its

. contacts I, 2 and 4, 5are opened and condensers C1 and C1 in lines a and b are cut out from the through-circuit during each train of impulses, due 7 to the fact, that relay F11 is slow acting and does not release the contacts between the consecu-- tive impulses of each train of impulses. In the same way, relay F3, which is slow acting, does not release the contacts during each train of impulses when contacts 2, 3v of relay F1 are opened.

After the loop-circuit is established in the aforementioned way, the rectifier A: can in known ways be short-circuited or disconnected or also be kept in the impulsating loop circuit. When the talking circuit is finally'established the rectiflers, of course, must not form part of the feeding or talking circuit, as otherwise talking would not be possible. The talking circuit beyond group-selector GV1 on exchange S1 is not shown on the drawings, as not forming part of this invention.

When the calling subscriber of exchange S1 restores his receiver to the hook, the relays F1, F1,

F4, G11 and G1: are released.

Upon release of relayF4, F5 will return to normal position and upon release of relay Fa, Fe will also return to normal.

Relay F1 is used for supervision purposes, no armature or contacts being shown on the drawings, as these features do not form partof this .back to'each other, which circumstance prevents the current to flow in either direction when the said line L is idle, which is one of the principal subscriber in exchange 51 reaches the line L in.

the following way: When the subscriber lifts his receiver from the hook, the subscriber's line circuit (not shown) is closed and operates a line relay. In known manner the subscriber dials a number corresponding toexchange S: and, over group selector (GV) multiple bank, he is directly or indirectly connected to relay F1, which receives the same number of impulses dialled by the subscriber. Relay F1 is thus closed over the following circuit: Grounded pole of battery B1, upper winding of relay F1, line a over GV multiple bank to subscribersinstrument and back (not shown on the drawings), over line b, lower winding of relay F1, negative pole of battery B1.

Armature of relay F1 is operated and contacts 2, land 4; 5 are closed. Through the closing of said contacts 2, 3 relay F: is operated in the 5 following way: Ground pole of battery B1, contacts 2, 3 of relay F1, winding of relay Fa, negative pole of battery B1. Upon operation of relay F3, its armature is operated and the following contacts are closed: 2, 8 and 4, I and 6, I.

10 When contacts 6, 1 of relay F3 are closed, relay F4 is also operated by the following circuit: Ground pole of battery B1, ,contacts I and 6 of relay Fa. winding of relay F4, negative pole of battery B1. F4 will thus be energized and its arma- Upon the closing of the contacts 2, 3 and 4, 5 of relay F4, the following circuit is established: 7

Ground pole of battery B1, rectifier R3, upper winding of relay G11, contacts I, 2 of relay'F14,

upper wire of line L, contacts 2, 3 of relay F4, cong};

tacts 2, 3 of relay F3, upper winding of relay F1, contacts 5,4 of relay F1, contacts 6, 5 of relay F4, lower wire of line L, contacts 5, 4 of relay F14, lower winding of relay G11, rectifier R4, negative pole of battery Ba.

'30 The call to exchange S: is now completed over the aforementioned loop circuit, and the transmission of impulses can take place in a similar manner to that described in connection with Fig; 1.

1 3 After the loop-circuit is established as aforementioned, the rectifiers R3 and R4 may be disconnected from the loop inknown ways, because no disturbing currents can do now any damage to the impulses to be sent. They may also be kept in 40 the impulsing loop-circuit, if so desired, depending upon the characteristics of the line, the relays, etc.

When the talking circuit is finally established, the rectifiers shall not form part of the feeding or talking circuit. In such case, the rectifiers R: and

R4may be short circuited, or a special feeding current may be supplied to the talking circuit (not shown on the diagram) through retardation coils.

restores his receiver to the hook, the relays F1, Fa F4 and G11 are released. .Upon release of relay Fa, the relay Fa will also return to normal position.

When the calling subscriber of exchange 81 It should be noted that when the line is idle,

similar conditions exist at bothexchanges, i. e. batteries B1 and B2 are connected through the respective lower windings of relays G1 and G11 and lower wire of line L in series with rectifiers R: and R4, respectively, with, the rectifiers connected to each other in opposed operating position.

In the same way, rectifiers R1 and Rs are connected back to back; opp sing current in either direction. ground and through the upper windings of relays G1 and G11 respectively to the upper wire of the Both rectiflers are connected ytQ the rectifiers, no disturbing currents can enter the line L or cause false operations of relays G1 or G11. 1

Fig. 3 shows an open-circuit Morse telegraph system, in which the receiving electromagnets are normally connected in series with rectifiers in opposed operating positions to ground, whereby disturbing currents cannot enter the system and operate the electromagnets. When key K at the left-hand end of the line-L is depressed, current flows from ground through rectifier Al in the direction of the current, electromagnet RI, key Ki line L, key K and battery B, to ground. 'Electro- .magnet RI becomes energized. Disturbing current in the opposite direction which woulchtend to weaken the operating current through the line, cannot pass through the rectifier AI. Disturbing currents flowing in the same direction as the operating current would cause no harm because the 'electromagnets can be'more easily adjusted for heavy currents than for weak currents.

If, however, parasitic currents flowing in the same direction as the operating current should enter the closed-circuit Morse system and if these currents should be too strong for proper operation, the polarity of the operating batteries and the polarities'oi' the rectifiers may be changed so as to bar such currents.

When protection against parasitic currents is needed, the prevailing direction of disturbing ground currents should be observed in the neighborhood and the polarity of operating current sources and rectifiersso chosen as to prevent the entrance of the disturbing currents into the system.

Fig. 4' shows a closed circuit Morse telegraph system in which rectifier A is connected in series with ground at one end of line L. This rectifier bars disturbing currents which would otherwise be 01' such magnitude and direction that the normal operating current would be cancelled,

, causing the deenergization of the eiectromagnets station A and rectifiers A2, B2, C2 and D2 at station 3. The rectifiers are connected to both the positive and the negative current sources and to both artificial line networks in such a manner that ground currents opposing the operating line currents cannot enter the system. The pole changers change the direction of the operating current at each end 01' the line at the same'time as the artificial networks with their rectifiers change position with respect to the line.

The principle of operation is the same as that of the well-known double current polar diilerential duplex system. When both, keys K and Kl are idle, the armatures oi the pole changers P01,

and PC: rest against their rear stops, which are connected to the positive generator terminals of generators V1 and V4, respectively. No current traverses the line wire. At stationA the current divides at m, one part traversing winding 0. and relay P, and the other part traversing winding b, both currents then combining at point 10 to flow through the artificial line ALI and rectifier Ci to ground, GI and back to the other generator terminal. At station B the current divides at the point 111., one part traversing winding 0 and relay Pl and the other part traversing winding d, both currents then combining at the point z, to flow through the artificial line AL and rectifier C2 to ground, and back to the other generator terminal. The direction of the currents through the two relays is such that the relay armatures will not be attracted to close the local sounder circuits S and Si, respectively.

The rectifiers Cl and C2 will prevent the passage of disturbing ground currents into the line via the associated artificial lines ALI and AL4. The rectifiers Aland A2 will prevent both under idle and operating conditions disturbing ground currents from passing through the generators in a direction opposite to that of 'the regular current.

Should a parasitic'ground'current flowing in the same direction as the normal current enter, e. g., from ground G via generator V1, rectifier Al, point m, through both windings of retardation coil J, relay P, line L to point 1/ and then through relay P1, point z, artificial line AL4, rectifier C2 and back to ground, relays -P and PI will not be disturbed, since the parasitic currents have the same direction as the local currents.-

When the key K is depressed, one contact of pole changer PCi connects with the negative generator (V2) terminal and the other contact connects with the artificial line AL2. Consequently, more current flows over the line than through either one of the artificial lines, and this current flows from u to :r. The line current enter ing at the point 1 is made up of the current flowing through the coil 0 and that coming from z through the relay PI. The direction of this current is such that the lower pole of the relay will be more strongly magnetized than the upper pole and, consequently, its armature closes the sounder circuit SI.

At station A the current divides at point :r, andthe part which traverses the relay P flows in a direction to magnetize the upper pole more strongly than the lower pole.- This relay will not close the sounder circuit S. Thus, the depression of one key controls the operation of the distant relay and sounder.

Rectifier C2 at station B and rectifier BI at station A will prevent disturbing ground currents from entering the system via artificial line AL4 or via generator V2. Should, however, a parasitic current enter the system via generator V4, passing rectifier A2 at station B, said current will do no harm to relay Pl at station B,

nor to relay P at station A, since the direction of said current is the same as that of the oper-- ating currents. of said relays. Rectifier Di now in circuit is in series with artificial line ALI at station A and will oppose any current coming from station B, while local current will flow as follows: Ground GI, rectifier DI, artificial line AL2, make contact of pole changer PCl, point to, winding b in parallel with relay P and winding 'a, m, minus contact of the pole changer, generator V: to ground. Should a parasitic current enter via'ground GI, rectifier DI and artificial line AL2, it would be possible in this case that such an opposing current could pass through the relay P, the line L relay Pl, artificial line A14 and rectifier C2 to ground. Owing to the triple resistance of this line circuit and the heavy operating current encountered, there is small probability of the stray ground current being able and PI.

It both keys are closed, the armatures 01 both generator terminals V: and V: respectively, and

'pole changers will be in contact with the negative to influence the normal operation of relays P no current will flow over the line. Currents will now flow through relay Pl from 2 to u and through relay P from w to :c, and their directions are such as to magnetize ,the lower poles -more strongly than the upper poles. The relay armatures will close both sounder circuits. Although each relay is operated by its local battery, its

action is controlled entirely by the distant key.

* systems using direct operating currents, particularly for use during non-operating circuit condition, in order to oppose disturbing currents. In each case one must determine the location of said rectiflers. They may be used in quadruplex using diplex and duplex, automatic telephony, printing telegraphy, ticker and printing telegraphs, railway and block signal systems, submarine telegraphy, etc'.

The most important advantages produced by the present invention are as follows:

Both direct and induced disturbing currents on a tnmk line during non-operating and oiltrailic conditions are eliminated; erroneous calls on the line are eliminated: distortion of sgnaling impulses ceases; erroneous disconnections of stations from the trunk line during service conditions are avoided.

When using the present invention, the trunk lines between automatic exchanges for instance, will be simpler and cheaper than before; the number oitreiays will be less and the circuit sig-' naling diagram simpler.

What we claim is:

-1. In a direct current signaling system, two

' stations, each thereof including a grounded current source, a two-way undivided trafllc trunk line interconnecting said stations, signaling means at each station allowing current to pass from said current source over the trunk for transmitting signals over said trunk, rectifier-s associated with said stations, receiving means .connected in series to a rectifier and to at least .one pole or the said grounded current source and to said trunk line at each station, said rectifiers being disposed at said stations and connect; ed-in opposed operating positions and in series with the same line wire, in order to suppress the effects of disturbing currents in both directions -on, the line and receiving means.

2. The system claimed in claim 1, in which each station comprises a telephone exchange. a common battery associated therewith and connected to ground, means for establishing a metallic loop circuit between said exchanges over said trunk, transmitting means disposed at the called exchange,'said transmitting means including the said current source and serving for transmitting signals from said current source over said trunk,

5. The system claimed in claim 1, for duplex signaling, in which said trunk comprises a single line wire with ground path, a positive and a negative current source at each station, rectiflers disposed in series with said current sources, means including pole changers at each station, said means being associated with the trunk and said current sources to connect said rectifiers together with their associated current sources 'with and disconnect the same from the trunk, whereby during current impulses said rectifiers permit the resulting operating current to flow in the proper direction, while the flow of disturbing extraneous currents is barred under non-operating conditions of the respective receivers.

6. The system claimed in claim 1, in which said trunk comprises a singleline wire and a ground return path, associated with a duplex signaling system, a positive and negative current source disposed ,at each station, pole changers for controlling the iiow of operating currents from the positive and negative poles of the current sources in both directions over the trunk, balancing networks at each end of the trunk connected in series with the rectiflers to allow the operating current to flow in the proper direction, and prevent the flow of disturbing extraneous currents under non-operating conditions of the respective receivers, a pole changer at each station for connecting said rectiflers and said balancing networks to and disconnect the same from the system. means for operating said pole changers simultaneously and during the same periods as the positive and the negative current sources are connected to the trunk at the same station.

'7. The system claimed in claim 1, comprising a two-wire trunk line, and a rectifier connected to each wire at each station, each rectifier barring the flow of current in an undesirable direction.

8. The system claimed in claim 1, in which said trunk consists of a two-wire trunk line and in which, at each station, a grounded rectifier is connected with one wire of the trunk and a relay at each station to control the connection of said rectifier to and its disconnection from the system.

9. The system claimed in claim 1, in which said trunk consists of a two-wire trunk line, and in which at each station a grounded rectifier is connected with one wire and a further rectifier is connected in series with the current source and with the other wire of the trunk, and a relay at each station for controlling the connection between the associated rectiflers and the trunk wires.

10. The system claimed in claim 1, in which said trunk consists of a two-way single-wire trunk for duplex signaling between two telegraph ofllces, two grounded rectiflers being provided at each oflice, a balancing artificial line connected with each rectifier, and a pole changer, at each ofllce for alternatively connecting the two associated rectifiers and said balancing artificial lines with the said trunk.

11. The system claimed in claim 1, in which said trunk consists of a two-way single-wire trunk between two telegraph oflices, tour rectifiers disposed at eachoflice two grounded and the other two connected with alternate poles of the current source, and a pole changer asso-,

ciated with each end ot the trunk and having two contacts connected with the grounded rectiflers and two contacts connected with the other two rectiflers.

12. In a direct current signaling system, two

suppress, under non-operating conditions,'the et-.

fects of disturbing currents in both directions on ,the line and the receivers.

13. The system claimed in'claim 12, in which each station comprises means for disconnecting said rectifier from the trunk line immediately after the receipt of a call over the said trunk go line, 'and means for reconnecting said rectifier .as soon as the line is restored to normal.

14. The system claimed in claim 12, in which each station comprises means for disrfonnecting said rectifier from the trunk line immediately 25 after the receipt of a train of signal impulses over the said trunk line, and means for reconnecting said rectifier as soon as the line is restored to normal, to suppress disturbing currents through the receivers during the intervals be tween each signal 'impulse received. i

15.-In a direct current signaling system, two stations, a two-way trunk line interconnecting said stations, each station including a direct current source and receiving means connected to .said trunk line, said current source having one pole grounded, av relay switch for disconnecting said receiving, means from said trunk line, rectifiers associated with said stations, the receiving means comprising a relay, one winding 01' the latter being connected in series with a rectifier and to the grounded pole of said current source,

said rectifiers being disposed at the respet'ive stations and connected in opposed operating positions, whereby signaling currents .between the stations are admitted, whereas, during non-oper-- ating conditions, disturbing currents in both directions are barred from the line and the said relay winding.

, 16. In a direct current signaling system, two stations, a two-way trunlrline comprising a plurality of line wires interconnecting said stations, each station including a direct current source and receiving means connected to said trunk line, a relay switch at oneof said stations for disconnecting the receiving means at "said station'from said trunk line and establishing a metallic loop circuit from the receiving means of the other station over said trunk line, rectifiers associated with said stations, the receiving means at each station comprising a relay with two windings each thereof in series with a rectifier and the grounded current source, said two windings being connected to difierent poles of said current source',-said rectifiers being disposed at the respective stations and connected in'opposed operating positions and in series with the same line wire and the corresponding relay windings at both stations, whereby signaling currents between the stations are admitted, whereas, during non-operating conditions, disturbing currents in 1 both directions are barred from the line and said relay windings.

S'I'EN DANIEL VIGREN.

'. EELGE ROST. 

